Biology of the garden pebble moth and damage caused to brassicas

Garden pebble moth larvae feed on the foliage of brassicas, sometimes mining into the hearts. Larvae feed within leaf folds beneath protective silk webbing, which can cause contamination issues.

• The larvae hide themselves within plant foliage, so infestations may not be detected until damage has already occurred

Scientific name: Evergestis forficalis

Adults have a wingspan of 25–30 mm. The forewings are yellowish–white, with brown veins and are covered with a series of oblique brown lines and shaded areas.

Eggs are shiny, oval and flattened. They are initially translucent before becoming yellow.

Young larvae are yellowish-green, but later become glossy pale green with yellowish mid-dorsal and lateral stripes.

Fully grown larvae are 18–20 mm long with a row of black spots along each side.

Nov–Apr: Pupae overwinter in the soil.

May–Jun: Adults (first generation) emerge and lay eggs in batches of about 20 on the undersides of leaves.

May–Oct: Larvae feed on the underside of leaves, frequently as a group within leaf folds and beneath protective silk webbing. The larvae pupate in cocoons in the soil.

Aug–Sep: Adults (second generation) emerge and lay eggs.

The second generation is the most damaging to brassica crops.

Non-chemical control

Several polyphagous predators attack this pest. The eggs or larvae may also be parasitised by certain species of wasp or fly, which eventually kill the larvae. The larvae continue to feed for some time after they have been parasitised, so crop damage is not reduced immediately.

To date, biological control with predators or parasitoids has not been investigated in the UK. It is not clear how susceptible the larvae are to products based on Bacillus thuringiensis (Bt).

Monitoring

Catch male moths in pheromone traps.

Thresholds

None established.

Insecticide resistance

None known.